Recently, there has been considerable interest in non-traditional measurement systems, such as wireless sensor networks. Wireless sensor networks generally comprise a plurality of sensor nodes that are each operable to perform some measurement and communicate wirelessly. Sensor nodes are commonly equipped, for example, with sensor(s) (or “measurement devices”), local storage, a processor (e.g., a central processing unit (CPU)), and wireless (e.g., radio) communication facilities. Such sensor nodes are typically small (e.g., include micro-sensors), and typically have short-range wireless communication capability.
Generally, the sensor nodes have one or more of the following characteristics: a) the nodes are desired to operate for extended periods of time on battery power; b) the nodes have limited computation, memory, and communication capability often due to power constraints; c) the nodes typically communicate using short-range wireless communication; d) the nodes are commonly installed in remote or other environments that preclude normal communication and control of the devices; and e) the nodes are often inexpensive. Sensor nodes are generally expected to be long-lived (deployed for years), untethered (both in terms of communication and power), and unattended (and so are capable of self-configuring and self-adapting). Wireless sensor networks often comprise a large number of sensor nodes that are deployed within a physical environment of interest, and such sensor nodes may measure aspects of the physical environment in great detail.
Sensor nodes may be deployed in a wireless sensor network in different ways. In one technique, ad-hoc deployment (e.g., random scattering) may be used, wherein sensor nodes are dropped with no particular plan or pre-defined arrangement. For example, initial deployment may involve dropping sensor nodes from an aircraft into an area of interest at random. The resulting wireless sensor network is referred to as an “ad-hoc” network (and is also sometimes referred to by other terms, such as “scatter nets” or “pico nets”). After being deployed in this ad-hoc manner, the sensor nodes interact with each other to establish a communication network among themselves. In another deployment technique, sensor nodes are specifically placed in desired locations, wherein the sensors may be precisely positioned relative to one another.
While individual sensor nodes may have limited functionality, the global behavior of the wireless sensor network can be quite complex. Thus, the functionality of the whole may be greater than the sum of its parts. This may be achieved, in part, through data fusion (i.e., the process of transforming and merging individual sensor readings into a high-level sensing result). That is, sensor nodes may both sense/measure a characteristic of their local environment and communicate locally with other local sensor nodes to construct semantically rich conclusions about that local environment.
Sensor nodes may have the capability of measuring at least one characteristic in their environment, such as detecting ambient conditions (e.g., temperature, humidity, movement, sound, light, or the presence or absence of certain objects). Many potential applications of wireless sensor networks exist, including as examples physiological monitoring, environmental monitoring (e.g., monitoring air, water, soil, chemistry, etc.), condition-based maintenance, military surveillance, precision agriculture, geophysical monitoring, transportation, monitoring of business processes (e.g., factory instrumentation and inventory tracking), animal monitoring (e.g., detecting the presence of animals), habitat monitoring, and/or measuring various other types of events.
Typically, the primary resource constraint of nodes in sensor networks is energy. Because many sensor networks deploy sensor nodes that are battery powered and that can scavenge only a small amount of energy from their surroundings, limited battery power is one of the major hurdles in achieving desired longevity of network operation. Reducing power consumption in sensing and subsequent data collection has been a topic of extensive study. The primary energy consumer in most wireless network sensors is the wireless (e.g., radio) transmissions.
Wireless sensor networks may collect a tremendous amount of detailed measurement (or sensed) data about their local environment. Such data may be communicated to an application that is located remote from the wireless sensor network. In some cases, a local high-powered (or long-range) communication device (or “data collector”) may be used to collect data from the sensors and relay that data to the application and/or to provide information from the application to the sensors.